This invention relates to a method of making fiber optic couplers that can withstand relatively wide temperature excursions and mechanical influences such as pulling on the optical fiber pigtails extending therefrom.
Fiber optic couplers referred to as "fused fiber couplers" have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores.
Various coupler properties can be improved by encapsulating the coupling region of the fibers in a matrix glass to form an "overclad coupler". Portions of the fibers to be fused are inserted into a glass tube having a refractive index lower than that of the fiber claddings. The tube has a longitudinal bore, each end of which is connected to the tube end surfaces by a funnel that facilitates the insertion of the fibers. The tube midregion is collapsed onto the fibers; the central portion of the midregion is then drawn down to that diameter and coupling length which is necessary to obtain the desired coupling.
After the midregion of the coupler has been collapsed (see FIG. 2), the fibers in the collapsed region are fused to the tube glass. The tube glass usually flows around and completely encases the fibers as shown in FIG. 2a, although it may be desirable, for certain applications, to maintain narrow, elongated open regions or air lines adjacent the fibers in the coupling region. Progressing from the collapsed midregion toward the uncollapsed portion of the tube bore, a cross-section through the preform reveals small air lines that begin to form at points 28 adjacent the fibers. At a greater distance from the collapsed midregion, the air lines enlarge as shown in FIG. 2b wherein the fibers are connected to the tube by narrow bridging regions 29 and to each other by narrow bridging regions 30. At some distance from the collapsed midregion, the fibers are separated from each other and from the tube as shown in FIG. 2c. The ends of the narrow bridging regions 29 and 30 are potential fiber break initiation sites. When an axial pulling force is applied to a fiber, breakage often occurs in this region of the coupler. This region, which is referred to herein as the "weakened region" of the fibers, usually occurs somewhere within the distance d which is about 5 mm from the end m of the collapsed midregion.
After the coupler has been stretched and cooled, a drop of glue is applied to each funnel to increase the pull strength of the fiber pigtails. Air trapped in the bore tends to keep glue in the funnel from penetrating into the bore. Techniques for increasing the depth of penetration of the glue into the uncollapsed portion of the tube bore are disclosed in U.S. patent application Ser. No. 07/913,622 (Berkey et al. 26-7) filed Jul. 16, 1992. In a first embodiment, glue is applied to the funnel. Before the glue is cured, a sufficient period of time is allowed to elapse to permit beads of glue to flow by capillary action between the fibers and the adjacent portion of the wall of the bore portion. The glue preferably flows at least 3 mm into the bore portion beyond the bottom of the funnel. The glue is then cured. In a second embodiment, a hollow filament is inserted into the uncollapsed bore portion, and a vacuum is applied to the filament. Glue, which is applied to the funnel, is drawn into the uncollapsed bore portion due to the evacuated condition thereof.
Under severe thermal cycling, the weakened region of the fibers has been known to break, primarily because of a thermal coefficient of expansion mismatch between the glue and the glass coupler components. Because of the angle of the funnel walls with respect to the bore axis, the thermal expansion mismatch causes the glue in the funnel to expand longitudinally outwardly and pull the fibers embedded therein away from the collapsed midregion, thereby stressing the fibers. If the uncollapsed portion of the tube bore is fully filled with glue, the fibers can be stressed by the presence of voids or other azimuthal inhomogeneities. Moreover, even a thin bead of glue that has wicked down the fiber to the bridging region may weaken the fiber during thermal cycling. When such glue is situated in the narrow bridging region 29 between the fiber and tube wall, it can act as a wedge as it expands due to an increase in temperature. If a fiber breaks away from the tube wall, i.e. the bridging region fractures, the damaged region of the fiber becomes a flaw from which crack propagation will initiate if the fiber is subjected to tensile stress.
It is therefore desirable to cause glue to flow a sufficient distance below the funnel to provide adequate pull strength, but to prevent the flow of glue over the bridging regions. Process reproducibility would be enhanced if the glue could be made to consistently flow to some predetermined narrow region below the tube endface. For example, the region could extend to some point in the tube bore a given distance above that region where fiber begins to bridge to the tube, i.e. the end of bridging regions 29. The techniques disclosed in the aforementioned Berkey et al. patent application do not consistently cause the glue to extend to a predetermined region within the uncollapsed bore.
When making a fused biconically tapered coupler such as that disclosed in U.S. Pat. No. 5,013,117, two or more fibers are fused together and stretched to form a coupling region. The resultant coupler, which is not supported by an overclad tube, is extremely fragile and must be attached to support means. For example, the ends of a coupler can be glued to a substrate. Some of the glue can wick between the fibers toward the coupling region. If the glue reaches the bridging region where the fibers begin to fuse together, it can cause failure during thermal cycling for reasons discussed above. The reproducability of the method of making this type of coupler would also be enhanced if the glue could be made to extend to a region that is sufficiently spaced from the bridging region.